Biological signal transduction involves biochemical, biophysical, and/or biomechanical processes by which a cell converts one type of signal into another. In the course of such transduction, a cell typically senses and responds to an external stimulus (such as a hormone). This often initiates a sequence of biochemical reactions associated with various types of molecules present in the cell. Examples of such molecules include receptors, second messengers, enzymes, transcription factors, DNAs, and mRNAs.
The task of finding the relevant signaling pathways and the network of molecular interactions responsible for a particular signaling event can be a difficult one.